The present invention is directed generally to an electric discharge source for generating radiation, comprising a system for producing a fluid filament or jet of controlled dimension in combination with coaxial electrodes, and particularly to a electric discharge source for generating extreme ultraviolet radiation.
Integrated circuits are typically manufactured using lithographic processes. Radiation (i.e., light) is caused to interact selectively with a photosensitive resist material deposited onto a substrate in such a way that a pattern or image from a mask is produced on the resist material. The resist material is developed and the pattern is transferred onto the substrate by etching.
In order to satisfy the demand for an increasing number of transistors contained on an integrated circuit it has become necessary to replace the present design rule of 0.5 μm by design rules that require feature sizes of 0.25 to 0.18 μm and significant effort is presently being put into achieving 0.1 μm resolution. However, as the feature size decreases, the wavelength of light required for submicron resolution decreases (for a design rule of 0.1 μm light with a wavelength of about 13 to 15 nanometer (nm) is preferred).
Extreme ultraviolet radiation (EUV), i.e., radiation having a wavelength in the range of 3.5–15 nm, can be produced by a variety of sources including laser produced plasma, synchrotron radiation, high-harmonic generation with femto-second laser pulses, discharge-pumped x-ray lasers, and electron beam driven devices. Of these, a laser produced plasma has been shown to be an efficient source of EUV. In particular, a plasma produced by directing a laser at a target composed of frozen pellets of krypton or xenon or condensed xenon or krypton gases as they expand through a supersonic nozzle into a vacuum chamber, such as disclosed in U.S. Pat. No. 5,577,092 “Cluster Beam Targets for Laser Plasma Extreme Ultraviolet and Soft X-ray Sources”, issued to Kubiak et al., has been shown to be a stable source of EUV radiation. While laser produced plasmas are able to convert between 1 and 4% of the incident laser power into EUV light, the cost and complexity of the integrated laser and Xe light source is high compared with conventional light sources. Moreover, it is not clear that the laser-produced plasma can achieve the required output of 30–60 W of EUV source power at acceptable manufacturing costs. Furthermore, because laser plasma systems exhaust large quantities of Xe gas into the target chamber a gas recirculation system is required to recapture the excess Xe in order to maintain a low background pressure in the target chamber and keep Xe costs down.
In an attempt to address the issues of complexity and high cost as well as achieving the desired 30–60 W EUV source power, efforts have been directed to discharge sources for producing EUV radiation and particularly capillary discharge sources such as those disclosed in U.S. Pat. Nos. 5,499,282 “Efficient Narrow Spectral Width Soft X-ray Discharge Sources”, U.S. Pat. No. 6,188,076 “Discharge Lamp Sources Apparatus and Method, and 6,031,241, “Capillary Discharge Extreme Ultraviolet Lamp Source for EUV Microlithography and Other Related Applications”, issued to Silfvast et al.
A capillary discharge source, such as that show n in U.S. Pat. No. 6,188,076 and illustrated schematically in FIG. 1, generally consists of an insulating material 110 having an open capillary channel 115 filled with a gas that allow s for electrical conduction within the capillary. The capillary channel can be cylindrical shape and can have a diameter in the range of 0.5 to 3 mm and be about 0.5 to 10 mm long. Electrodes 120 and 125 are attached to insulating material 110 on either side of capillary channel 115. A high voltage pulse is applied to the electrodes to generate a plasma 130 thereby providing a beam of radiation whose wavelength is determined by the composition of the plasma. The radiation, emitted along the capillary axis, can be collected and directed for some purpose such as EUV lithography.
While capillary discharge sources are compact and provide a relatively low cost and efficient source of EUV radiation they do suffer from some significant drawbacks. In particular, debris generated by interaction of the plasma with the walls of the capillary and the electrodes is ejected from the capillary along the axis of the capillary. Consequently, mirrors and other light collecting optics that are directly in line with or proximate the capillary discharge source become quickly coated with the ablated material. This coating of ablated material reduces the effectiveness and lifetime of light collecting optics. Pre-processing methods, such as exposure to discharge current pulses and laser heat treatment, have been proposed to limit debris generation (cf. U.S. Pat. No. 6,188,076). However, these methods are time consuming and may not be successful enough to lower ablation levels to a low enough level to avoid coating of sensitive optical components.
Moreover, the capillary itself must be able to dissipate enough of the heat load generated when operated at high repetition rates to maintain its structural integrity. Thus, the thermal conductivity of the capillary material must be high enough so that the capillary can withstand the heat load, limiting the choice of capillary material. Furthermore, a discharge source for EUV lithography must be capable of operating in and compatible with a vacuum atmosphere.